1. Field of the Invention
This invention relates to the field of Disk Drive Suspensions. More particularly, this invention relates to the field of a Hard Disk Drive Suspension resistant to Pitch Static Attitude (PSA) changes due to variations in humidity and temperature.
2. Description of Related Art
FIG. 1 shows a typical magnetic hard disk drive (HDD) 10, such as is known in the prior art and to which the present invention may be applied. The HDD 10 generally uses a spinning storage medium 12 (e.g., a disk or platter) to store data. A suspension 14 is mounted to a support arm or actuator arm 16, which is rotated by a voice coil motor (not shown).
As data densities and hence data track densities increase in response to consumer demands for more and more storage, it becomes increasingly important to keep the head slider which is mounted at the end of the suspension 14 over the correct portion of disk 12, and aligned with the proper pitch and roll attitude. The pitch and roll attitude of a suspension including its head slider when not interacting with a spinning disk is called the pitch static attitude (PSA) of the suspension. The PSA of a suspension is typically finely adjusted at the time of its manufacture to the desired PSA. Any changes in the PSA over come time from the factory-adjusted PSA can adversely effect the disk drive's performance. In an extreme case, a drastic PSA change could result in disastrous failure of the disk drive.
Suspensions typically have a stainless steel support substrate or support layer, and a flexible circuit that typically includes an insulating layer such as polyimide, a plurality of copper or copper-alloy (hereafter, simply “copper”) conductive metal signal traces to carry data read-write signals between the head slider and pre-amplifiers that are located off the suspension, a gold layer over the copper signal traces, and an insulating protective coverlayer such as acrylic or polyimide over the circuit to protect against oxidation and short circuiting of the signal traces.
One problem with prior art suspensions is the sensitivity of the PSA due to changes in temperature and humidity. The materials used for the insulating layer, which is typically polyimide, and for the coverlayer, are typically hygroscopic, meaning that they absorb water from the atmosphere. When those materials absorb water from the atmosphere they expand, and when they release water to the atmosphere they contract. In contrast, the metal support layer of the suspension, which is typically stainless steel, and to which parts of the circuit are laminated or otherwise adhered, is non-hygroscopic. The copper signal traces are also non-hygroscopic. When a suspension is exposed to increasing humidity, therefore, the polyimide and the coverlayer expand, while the stainless steel support layer to which it is adhered and the copper signal traces do not expand.
FIGS. 2-3B illustrates this situation. In these figures, a first non-hygroscopic layer 20, such as stainless steel, has a second and hygroscopic layer 22 such as polyimide over it. On top of that is a third and non-hygroscopic layer 24, which could be a copper signal trace. In a relatively dry condition shown in FIG. 2 and in cross section FIG. 3A taken along line 3-3, the structure lies flat.
FIG. 3B shows this structure under increased humidity conditions. In this figure, hygroscopic layer 22 has absorbed moisture from the atmosphere and has volumetrically expanded. Both layers 20 and 24 are non-hygroscopic and have not expanded. The copper layer 24 is thinner, narrower, and softer than stainless steel layer 20, and hence presents less resistance to expansion than does stainless steel layer 20. The result is that the structure bows downward as shown.
When the structure is polyimide on stainless steel with no copper on top, the structure will bow downward as in FIG. 3B. Hard disk drive suspensions typically include flexible circuits that have suspended portions, i.e., portions that comprise copper or copper alloy signal traces on polyimide with no stainless steel support layer below. When the structure is copper on polyimide with no stainless steel below, the structure will bow upward. In general, therefore, in response to increased humidity, a suspension circuit will have areas that bow upward, areas that bow downward, and areas that may be warped and wavy, with the locations and severity of the bowing and warping changing as the humidity changes. The severity of the changes will depend on the differences in the coefficients of hygroscopic expansion of the materials, and the percent humidity change.
A similar bowing and warping effect occurs due to changes in temperature, due to the differences between coefficients of thermal expansion in the materials used.
The ways that a suspension responds to humidity and temperature variations are sometimes referred to as the hygrothermal characteristics of the suspension. Hygrothermal effects negatively affect the PSA of a suspension, and hence the performance of the disk drive.
Various solutions have been proposed to the problem of hygrothermal effects in a suspension.
U.S. Pat. No. 5,982,584 issued to Bennin et al. proposes a suspension in which a first and supported portion of the circuit includes copper on polyimide on stainless steel, and a second and suspended portion of the circuit in serial with the first portion which includes copper on polyimide without any stainless steel, such that in response to increased humidity the first part bends upward and the second part bends down, with the two bendings purportedly counteracting each other.
U.S. Pat. No. 6,381,100 issued to Bennin et al. is similar to Bennin '584 but proposes a parallel solution as opposed to a serial solution. More particularly, Bennin '100 proposes a suspension in which a first and supported portion of the circuit includes copper on polyimide on stainless steel, and a second and suspended portion of the circuit in parallel with the first portion that includes copper on polyimide without any stainless steel, such that in response to increased humidity the first part bends upward and the second part bends down, with the two bendings purportedly counteracting each other.
U.S. Pat. No. 6,515,832 issued to Girard proposes adding distally extending non-water absorbent appendages to the suspension. The appendages resemble copper horns at the distal end of the suspension. The appendages purportedly act as deformation inhibitors.
U.S. Patent Publication No. 2009/0109568 by Hernandez proposes that a head gimbal assembly (HGA) be fabricated such that thermally induced variations in the slider profile (crown) are counterbalanced by controlled thermally induced changes in the PSA of the suspension. In an embodiment, Hernandez's structure includes a thin layer of diamond-like carbon (DLC) as a thermal element that is deposited on the backside of the outriggers of a suspension ring gimbal.